1. Field of the Invention
The present invention relates to absolute-value measurement of optical information associated with scattering and absorptive constituents in a scattering medium by utilizing modulated light and, more particularly, to an apparatus for measuring optical information in the scattering medium, capable of measuring an equivalent scattering coefficient, an absorption coefficient, and the concentration of a specific constituent in the scattering medium, their time rate changes, spatial distributions, and the like, and improving measurement precision, and a method therefor.
2. Related Background Art
Light does not propagate straight in a scattering medium because the light is scattered and absorbed at random. The total amount of light is not reduced in a scattering medium whose absorption is zero. However, light propagates in the medium at random in a zig-zag manner because the light is scattered by the scattering constituent at random. In this case, an average optical pathlength through which light propagates without the influence of scattering is a reciprocal number of an equivalent scattering coefficient (to be described later). This reciprocal number is called a mean free path or mean diffusion length. This pathlength in a biological sample is about 2 mm. However, as an absorptive constituent is contained in addition to the scattering constituent in a scattering medium, random absorption occurs to attenuate light in accordance with the light propagation path length.
A well-known Lambert-Beer law is valid in the above scattering medium. According to this law, the absorbance or optical density of the scattering medium is proportional to the product of the molar absorption coefficients and the molar concentrations of the constituents and thickness of the scattering medium. This law is regarded as a basic principle in absorbance analysis.
In the absorbance measuring method, the attenuation coefficient by its definition is a sum of an equivalent scattering coefficient and an absorption coefficient. These scattering and absorption coefficients are processed as equivalent parameters. For this reason, it is impossible to separate influences of scattering and absorption and to accurately measure the influence of absorption, e.g., the absorption coefficient. To overcome this, the principle of two-wavelength spectroscopy is generally applied to this absorbance measuring method. More specifically, at least two appropriate light beams having different wavelengths and different absorption coefficients with respect to an absorptive constituent are used to measure the absorbance values.
In this case, it is assumed that the scattering coefficients or equivalent scattering coefficients at these at least two wavelengths are identical to each other or have a very small difference, if any. The influence of scattering is eliminated in accordance with the difference between the absorbance values derived from these at least two light beams, thereby obtaining the absorption coefficient or the concentration of the absorptive constituent.
According to this method, as can be apparent from the above measurement principle, a large error occurs due to the assumption that the scattering coefficients are equal to each other with respect to the light components having different wavelengths. In addition, the influence of scattering, i.e., the scattering coefficient itself cannot be measured. As techniques similar to this, a method of measuring an absorbance difference using two- or three-wavelength continuous, pulsed, or modulated light and a method using the above principle of two-wavelength spectroscopy in addition to the above method of measuring the absorbance difference are also available. These methods also have the same disadvantage as that of the above principle, and this disadvantage cannot be eliminated.
Strong demand has conventionally arisen for measurements of absorptive constituents in scattering media such as living bodies or improvements of measurement accuracy, and various efforts and attempts have been made. Major efforts and attempts are summarized as references.sup.1)-7) at the end of this section.
In these references, a common problem, i.e., a problem encountered upon application of the principle of two-wavelength spectroscopy to absorbance measurements including scattering, is posed. In addition, the following problems are also posed. In the following description, .sup.x) represents the reference number.
Tamura et al..sup.1) proposed the measuring principle of an oxyhemoglobin concentration and a reduced or deoxygenated hemoglobin concentration in accordance with a change of absorbance optical density with respect to incident light components having three different wavelengths. Optical CT construction using this principle is also attempted. However, this method has the above problem and poses another problem in which a measurement error is increased by the way of handling an optical pathlength upon a change in absorption coefficient in absorbance measurement.
References.sup.2-4) are attempts for time-resolved measurement where time-resolved output signal with respect to the incidence of pulsed light is used to measure internal absorption information. At this time, an output light signal upon incidence of the pulsed light on a scattering medium is a light signal output which has a wide time width caused by scattering and absorption and a long, gradually attenuated tail.
Patterson et al..sup.2) assumed a model of a uniform scattering medium to analytically obtain a light signal output in response to pulsed light incidence. A waveform representing a time change in intensity of the optical signal given by the formula defined by Patterson et al. matches a waveform obtained by an experiment using a uniform scattering medium. According to them, the absorption coefficient of an absorptive constituent constituting the scattering medium is given by a slope (differential value) obtained when the optical signal is sufficiently attenuated, i.e., when a sufficiently long period elapses.
According to this method, however, since the optical signal corresponding to a portion subjected to absorption coefficient measurement must be sufficiently attenuated, the S/N ratio of the signal becomes low, and an error increases. It is difficult to use this method in practice. In addition, a long period of time must elapse until the optical signal output is sufficiently attenuated, and the measurement time is inevitably prolonged.
To the contrary, Chance et al..sup.3) proposed a method.sup.3) of obtaining a slope at an earlier timing (when the light intensity is not sufficiently attenuated) to approximate an absorption coefficient with the slope value. According to their report, an error in a simple scattering medium such as a uniform medium is about 10%. However, there is no guarantee that the above waveform is monotonously attenuated in an actual living body having a complicated structure, and an error caused by an increase in DC light component is further added. In the above three references, the scattering coefficient cannot be measured.
Sevick, Chance, et al.sup.4) calculated an average optical pathlength of detected output light components from the barycenter, i.e., the average delay time of the waveform of an output signal obtained by Patterson et al. mentioned above, and confirmed dependency of the average optical pathlength on the absorption coefficient. They also attempted to measure an absorptive constituent localized inside the scattering medium from a change in average optical pathlength.sup.4).
The method of Sevick, Chance, et al. explicitly suggests that application of the concept of the average optical pathlength which depends on absorption allows measurement of absorption information in the scattering medium. However, the above average delay time can be obtained only after the output signal waveform becomes apparent as a whole. For this reason, measurement of the average delay time must be delayed until the output light signal having a long, gradually attenuated tail is sufficiently attenuated, thereby prolonging the measurement time.
According to this method, since the output light signal is obtained by time-resolved measurement, the improvement of measurement precision of time to improve measurement precision of the average optical pathlength undesirably causes a decrease in S/N ratio. Therefore, the measurement precision has a limitation. Signal processing for obtaining the barycenter is complicated, and an apparatus for performing time-resolved measurement is generally complicated and bulky, resulting in an impractical application.
On the other hand, Gratton et al. proposed a method.sup.5) utilizing light modulated with a sinusoidal wave in imaging of the interior of a scattering medium. This method utilizes coherent propagation of a photon density wave having a modulated frequency component in the scattering medium, as will be described in detail with reference to the operational principle of the present invention.
According to their report.sup.5), although a coherent photon density wave propagating in the scattering medium is confirmed in their experiment, optical parameters of a sample actually used in the experiment do not match the theoretically calculated values. This study is still in the stage of fundamental study. No detailed findings and means have been obtained for a method of calculating absorption and scattering coefficients as one of the objects of the present invention, a method of obtaining the concentration of a specific constituent, and the like.
Chance proposed a method and apparatus for determining the concentration of an absorptive constituent in a scattering medium utilizing modulated light in 1989 prior to the report of Gratton et al., and U.S. Pat. No. 4,972,331.sup.6) of this method was issued to Chance in 1990. According to the basic principle of this patent, an output signal upon incidence of modulated light on the scattering medium is detected and compared with a reference waveform (incident light waveform) to determine a quantitatively measurable parameter, an optical pathlength obtained in the time-resolved measurement mentioned earlier is calculated, and the concentration of the absorptive constituent is quantitatively measured.
This patent also discloses an application of the two-wavelength spectroscopy principle. This reference, however, uses two wavelengths to eliminate the influence of scattering. That is, Chance's patent proposes a technique for accurately measuring the optical pathlengths in accordance with a phase difference method. The Chance's patent is substantially identical to the conventional techniques described above and cannot eliminate the conventional drawback. That is, a scattering coefficient and a measurement error caused by a scattering coefficient difference in an application of the two-wavelength spectroscopy cannot be measured.
In addition, Chance also mentions that the phase difference obtained by the method of this patent is equal to the optical pathlength (barycenter of the wave) obtained in the time-resolved measurement and that logarithmic conversion of this phase difference is proportional to the concentration of the absorptive constituent of a scattering medium. The latter fact, however, is greatly different from the analytic and experimental results of the present invention, as will be described in detail later. The present invention does not require the determination of the optical pathlength. That is, the optical pathlength need not be calculated or measured.
In recent years, Sevick, Chance, et al..sup.7) systematically examined and analyzed the relationship between various parameters obtained in the time-resolved measurement method and a method (they call this method a frequency-resolved measurement method) utilizing the modulated light, including the analysis results of researchers except for Sevick, Chance, et al., and conducted experiments to verify their analysis results.sup.7). Most of the major conventional methods for measuring absorption information in a scattering medium are examined in this report, which is very convenient for us. The relationship between the parameters obtained by the time-resolved and frequency-resolved measurement methods is clarified. For example, when the modulation frequency is low, the phase difference obtained by the frequency-resolved measurement method is found to be proportional to the average optical pathlength obtained by the time-resolved measurement method. This is partially disclosed in the Chance's patent described above.
The report by Sevick, Chance, et al. describes detailed applications of the time-resolved measurement method which they have been studying. For example, the following method is described in detail. That is, parameters such as an average optical pathlength and an absorption coefficient are obtained from an output light signal obtained by the time-resolved measurement method. By using these parameters, the concentration and absorption coefficient of the absorptive constituent, the degree of saturation of hemoglobin (concentration of oxyhemoglobin with respect to the total amount of oxyhemoglobin and reduced hemoglobin), and the like in the scattering medium are obtained. These measurements employ the above-mentioned principle of two-wavelength spectroscopy, resulting in errors caused by the scattering coefficient differences. As described above, there are no new findings in this report, but this report can serve as a reference for understanding their idea.
Finally, to clarify the foundation of the present invention, differences between the present invention and the Chance's patent.sup.6) "Phase Modulated Spectroscopy" will be briefly described below. It is pointed out that, in the Chance's patent, although two-wavelength spectroscopy has various advantages in detection of changes in hemoglobin and cytochrome in a living tissue, as described in the part of the "background of the invention", the basic problem of a method of this type lies in that an animal model which allows elimination of hemoglobin to allow direct measurement of cytochrome must be referred to calculate the optical pathlength of a living body as an object to be measured because the optical pathlength is unknown.
It is then stated that a possible application of this method is a clinical study of time-resolved spectroscopy (TRS) using a picosecond optical pulse capable of quantitatively measuring a change in hemoglobin concentration upon determination of the optical pathlength and determining the actual concentrations of hemoglobin and cytochrome. In addition, it is suggested that when this time-resolved spectroscopy and continuous wave spectroscopy (CWS) are used together, the optical pathlength of photon migration can be calibrated to widen the application field according to Chance's patent. The above descriptions are assumed to indicate the importance of Chance's patent.
In contrast to a measurement algorithm closely associated with the above optical pathlength, the present invention utilizes a new measurement algorithm using a function having a form excluding the optical pathlength or a form excluding the optical pathlength as a variable. The optical pathlength naturally need not be calculated. In Chance's patent, the difference between the scattering coefficients with respect to different wavelengths causes a measurement error. To the contrary, according to the present invention, the influence of scattering can be eliminated because waves having at least two different frequency components are used in calculating optical information associated with absorption. The optical information associated with absorption can be accurately measured. That is, by using the waves having two different frequencies, the influence of scattering can be perfectly eliminated according to the present invention.
The first part of the "summary of the invention" of the specification of the Chance's patent describes that "when the carrier frequency is selected so that its time characteristics match the delay time of the photon migration during the period between the input to the scattering medium and the output therefrom, it is found that the principle of two-wavelength spectroscopy can be applied to time-resolved spectroscopic measurement". According to a description in the second half of the description of the third embodiment, since a carrier wave having a high frequency of 220 MHz is used in the apparatus of Chance's invention, measurement accuracy of the photon migration time between the input and the output of the characteristic time measured to be about 5 ns can be greatly improved.
The sensitivity of the disclosed apparatus is indicated to be about 70.degree./ns and 3.degree./cm of the change in optical pathlength. To apply the principle of two-wavelength spectroscopic measurement to the time-resolved spectroscopic measurement, the value of the carrier frequency must be selected such that the time characteristics of the carrier wave match the delay time between the input and output in photon migration.
According to a description in the second half of the last paragraph in the part of the detailed description, as the great advantage of the phase modulated spectroscopy, i.e., the method of his patent, it is emphasized that the optical pathlength can be obtained without any assumption. According to this description, when the optical output is exponentially attenuated and the photon migration length is large, the phase modulated spectroscopy can provide a function of emphasizing the delay time of about 5 ns, and his method is one of the most convenient embodiments of the time-resolved spectroscopic measurement.
Judging from the above description, Chance's patent is based on the findings obtained in the time-resolved spectroscopic measurement. The optical pathlength is determined by the phase modulated spectroscopy. At this time, the time characteristic, i.e., the period of the carrier wave is set almost equal to the delay time of photon migration, thereby apparently improving measurement accuracy of the delay time, i.e., the optical pathlength.
In other words, in the Chance's patent, a method of applying the principle of two-wavelength spectroscopic measurement to the time-resolved spectroscopic measurement is very effective. However, since determination or measurement of the optical pathlength by the time-resolved spectroscopic measurement is greatly limited due to the time resolution as the performance of the apparatus, complexity of the apparatus, high cost, and the like, the optical pathlength is measured by a simple phase difference method.
To the contrary, as described above, the present invention is based on a measurement method based on the entirely new concept and principle which are different from a conventional time-resolved spectroscopic measurement including the one disclosed in the Chance's patent or the combination of the time-resolved spectroscopic measurement and the two-wavelength spectroscopic measurement.
The optical pathlength measured in Chance's patent need not be measured in the present invention because the present invention is not based on the principle of time-resolved spectroscopic measurement. The present invention does not require selection of the frequency of a carrier wave required in the Chance's patent (according to the present invention, the carrier wave is expressed as a predetermined frequency component constituting modulated light), i.e., the present invention need not satisfy the condition that "the carrier frequency is selected so that its time characteristics match the delay time of the photon migration during the period between the input to the scattering medium and the output therefrom".
The present invention does not have any limitation concerning frequencies in principle. The principle of two-wavelength spectroscopic measurement can be applied in the entire frequency range. In this case, the difference between the scattering coefficients for different wavelengths need not be considered. That is, the present invention uses photon density waves having at least two different frequencies to eliminate the influence of scattering, as compared with the Chance's patent using two different wavelengths.
The differences between the present invention and the prior art have been clarified, and the inventive step, effectiveness, and importance of the present invention will be readily understood.